1. Field of the Invention
The invention relates to a magnetic resonance imaging method for reducing image errors in a magnetic resonance image determined from magnetic resonance signals which are generated in an object by means of measuring sequences, which object is arranged in a uniform, steady magnetic field, the measuring sequences comprising a phase encoding gradient which is varied from one measuring sequence to another. The invention also relates to a magnetic resonance imaging device for reducing image errors in a magnetic resonance image, which device comprises means for generating a uniform, steady magnetic field, means for generating measuring sequences in order to generate magnetic resonance signals in an object arranged in the steady field, and means for reconstructing the magnetic resonance image from the resonance signals.
2. Description of the Related Art
A magnetic resonance imaging method and device of this kind are known from U.S. Pat. No. 4,713,615. The cited United States Patent Specification describes the formation of gradient waveforms of inter alia phase encoding gradients by applying data in digital form, under the control of a computer, to a digital-to-analog converter (DAC) and by subsequently applying the analog output signal of the digital-to-analog converter to a gradient amplifier, an output of which is coupled to a gradient coil. Due to quantization errors in the digital-to-analog converter, it is not possible to adjust any desired amplitude of the phase encoding gradient, i.e. the amplitude of the phase encoding gradient must be rounded off to the nearest DAC value. As is known, for phase encoding gradient waveforms the gradient area should approximate a desired value as well as possible. According to said United States Patent Specification, inaccuracy in respect of gradient area is reduced by applying, at a different instant within the measuring sequence and in addition to the phase encoding gradient with the described amplitude quantization, a further gradient of comparatively short duration whose gradient area equals the area error in the previously applied phase encoding gradient as well as possible. Furthermore, it is to be noted that the aim is for low-cost DACs, i.e. converters having a comparatively low resolution in combination with an optimum effective phase encoding gradient area. As an alternative it is described that, using a low-cost DAC, a gradient having a desired area can be obtained as well as possible by applying a two-pole gradient for all desired areas, which gradient switches between a maximum and a minimum value, the timing of an edge thereof being varied upon transition from the maximum to the minimum value.
The alternatives described in said United States Patent Specification are based on an ideal DAC, i.e. differential non-linearity which occurs in practical situations and which still gives rise to inaccuracies in the adjusted gradient amplitudes is not taken into account. This is important notably for the phase encoding gradient, especially in orthogonal MRI methods where MRI data in the k-space is to be situated on a cartesian grid. When the data is situated on a grid in the k-space, an image containing a minimum number of image errors will be obtained after inverse Fourier transformation of the data. In the low-cost/low-resolution DACs proposed in the cited United States Patent Specification such differential non-linearity will be greater than in more expensive high-resolution DACs, because DAC manufacturers do not impose unnecessarily severe requirements on their DAC design. Furthermore, the application of an additional gradient will not be desirable for all measuring sequences and notably in a so-called spin-warp measuring sequence it will lead to undesirable prolongation of the sequence duration. The use of a two-pole gradient is not always desirable either. Furthermore, DAC instability and non-linearity and instability and offset of the gradient amplifier will also give rise to image errors.